Known cannulas (or puncture cannulas) for puncturing or piercing have a more or less strongly inclined wedge-shaped ground surface at an end portion of the cannula. The rear edge of the wedge-shaped ground surface forms a sharp blade. During the puncturing, a blade of this kind has the effect that constituent parts are punched out from the material that is to be pierced. When puncturing tissue for example, this is undesirable on account of the trauma caused. Moreover, the punching-out effect proves extremely undesirable if cannulas of this kind are used to puncture implantable port catheters. Such port catheters generally consist at least of an implantable capsule which has a hollow space for the removal and/or administration of a liquid such as blood, blood components or a medicament. The capsule is connected to a catheter that opens into a vessel or other treatment site. The wall of the implanted capsule directed toward the skin of the patient is provided in the form of a pierceable elastomer membrane, which is pierced by the cannula through the skin. The penetration of a cannula into the membrane of a port catheter causes leaks, since the rear edge of the ground surface punches elastomer material out of the membrane. This results in the formation of holes, which can no longer automatically close under the restoring force of the membrane material. This has the effect that the implanted capsule begins to leak after just a few piercing procedures. Moreover, punched-out elastomer particles can be carried off into the patient or can block the catheter.
The European patent EP 0 495 214 B1 describes a port cannula which can effectively reduce punching-out. The content of said document is incorporated in full, by reference, into the present application. The cannula described in said document is such that the blade formed by the rear edge is substantially concealed by the cannula tip and in this way can no longer cut. This is achieved principally by the fact that the cannula tube is bent laterally toward the piercing part and, in its further course, is hook-shaped and has a lumen opening ground tangentially into the concave recess of the hook-shaped tube, which lumen opening forms, at its rear edge, a blade that is strongly oriented inward on account of the design of the tube. The demands placed on a port cannula are substantially met by the port cannula described in EP 0 495 214 B1.